1. Field of the Invention
The present invention relates to a capacitance detection type sensor provided with an electrostatic discharge (hereinafter referred to as ESD) hole for preventing a breakdown thereof due to static electricity, and more particularly, to a capacitance detection type sensor suitable for detecting a fingerprint.
2. Description of the Prior Art
Recently, a highly information-oriented society has come, and thus the necessity of securing not only assets but also information has been increased. With such an increase of the necessity, systems for identifying individuals have come to be required. Among the systems as described above, there is a fingerprint detection apparatus for recognizing a fingerprint pattern to identify an individual.
In the fingerprint detection apparatus, an optical detection type fingerprint sensor is generally used. However, the optical detection type fingerprint sensor has a drawback that cost reduction thereof is difficult since a system becomes relatively large in scale. Accordingly, demand is increased for a capacitance detection type fingerprint sensor, in which a system is relatively simple, and cost reduction is enabled.
FIG. 1 is a plan view showing a configuration of a conventional capacitance detection type fingerprint sensor, and FIG. 2 is a schematic cross-sectional view showing the same.
On a semiconductor substrate 50, a large number of fine capacitance sensor electrodes 53 are formed in matrix arrangement. The capacitance sensor electrodes 53 are connected to a drive circuit 51 formed on the semiconductor substrate 50. Moreover, on the capacitance sensor electrodes 53, a cover film 55 made of an insulating material is formed.
ESD holes 54 are selectively formed in the cover film 55. The ESD holes 54 are formed in the vicinities of corner portions of the capacitance sensor electrodes 53, and on bottom portions of the ESD holes 54, ESD electrodes 56 are arranged. The ESD electrodes 56 are electrically connected to the semiconductor substrate 50 and grounded through the semiconductor substrate 50.
In the capacitance detection type fingerprint sensor thus configured, when a fingerprint is detected, first, a specified charge is filled in the capacitance sensor electrodes 53 from the drive circuit 51. Subsequently, as shown in FIG. 3, when a finger of a subject is brought into contact with the surface of the cover film 55, a potential of the capacitance sensor electrodes 53 are changed by a capacitance between the finger and the capacitance sensor electrodes 53. In this case, intervals between the finger and the capacitance sensor electrodes 53 differ depending on places because of unevenness (fingerprint) of the finger, and variation occurs in the potentials of the capacitance sensor electrodes 53. After the passage of a specified time, the drive circuit 51 detects the potentials of the capacitance sensor electrodes 53, and based on a result of the detection, an image showing a distribution of the potentials is created. As mentioned above, the potentials of the capacitance sensor electrodes 53 are related to the unevenness (fingerprint) of the finger, and therefore, the image showing the distribution of the potentials of the capacitance sensor electrodes 53 represents a fingerprint pattern.
Incidentally, in the capacitance detection type fingerprint sensor, it is necessary to bring the finger of the subject in direct contact with the surface of the cover film 55. In this case, there is an apprehension that static electricity (of several thousand voltage) charged in a human body may be discharged to the capacitance sensor electrodes 53 to break the cover film 55.
In order to prevent the breakdown of the fingerprint sensor due to the discharge, which is as described above, the ESD holes 54 are provided. Heretofore, a diameter of the ESD holes 54 has been 5 μm or more, and as shown in FIG. 1, the ESD holes 54 are arranged in the vicinities of the corner portions of the capacitance sensor electrodes 53. The static electricity charged in the human body is discharged to the ESD electrodes 56 in the ESD holes 54 preferentially rather than to the capacitance sensor electrodes 53. Therefore, destruction of the cover film 55 due to the discharge is avoided.
The inventors of the present invention conceive that the conventional capacitance detection type fingerprint sensor described above has a problem shown below. Specifically, moisture and salt are adhered onto the surface of the human finger. When the fingerprint is detected, it is necessary to bring the surface of the finger in contact with the surface of the fingerprint sensor. In this case, the moisture and the salt, which have been adhered onto the finger, enter the ESD holes 54. Usually, each ESD electrode 56 has a stacked structure of an aluminum film excellent in conductivity and a TiN film excellent in corrosion resistance. However, if cracks occur in the TiN film, the moisture and the salt enter the aluminum film therefrom, and the aluminum film becomes corroded.
FIG. 4 is a view showing a microscope photograph of the ESD hole. As shown in FIG. 4, in the conventional capacitance detection type fingerprint sensor, the cracks (portions surrounded by circles in the figure) frequently occur in the TiN film coating the aluminum film. When an accelerated test is carried out for the capacitance detection type fingerprint sensor as described above by salt spray (for 12 hours), the aluminum film is intensely corroded by the moisture and the salt, which have entered the aluminum film from the cracks of the TiN film. Consequently, wires and elements of the sensor are damaged, and a function as the sensor becomes lost.
Causes of occurrence of the cracks in the TiN film are not obvious. However, since a difference in thermal expansion coefficient between aluminum and TiN is relatively large, the cracks are conceived to occur due to occurrence of a large stress on an interface between the aluminum film and the TiN film, which is caused by heat during film forming and chip cutting. Moreover, the cracks are conceived to occur due to occurrence of a stress in the TiN film since grains in the aluminum film are moved by heat.
It is also considered that the ESD electrodes are formed of metal higher in corrosion resistance than aluminum. However, since the ESD electrodes are formed simultaneously with wiring of the drive circuit, if the ESD electrodes are formed of metal different from a wiring material, then there occurs a new problem that the number of manufacturing steps is increased to increase a cost of the product.